Various systems have been developed or proposed for utilizing laser beam energy for cutting, welding, engraving, etc. Surgical laser devices have also been developed for delivering laser energy from a laser to a site on or in a patient's body. In some applications, these surgical laser devices deliver laser radiation through a flexible, optical fiber from a laser to a target tissue.
Laser energy may be employed to produce a desired effect on tissue, including various types of human tissue. For example, laser energy may be employed to denature proteinaceous components and to cauterize, ablate, or cut tissue.
Typically, effects of a laser beam on human tissue (e.g., ablation, thermo-coagulation, denaturization, cutting, and the like) can be produced with pulses of laser radiation having, for example, at a wavelength of 2,100 nm, an energy density of about 300 mJ/mm.sup.2 incident on the target site tissue. The effects on the tissue are, of course, dependent upon the amount of incident radiant energy that is absorbed by the tissue and on the absorption efficiency of the employed wavelength. Relatively hard tissues, such as calcified atherosclerotic plaque or bone, require relatively high energy levels for ablation to be effective. Likewise, relatively high average power is needed for ablating a cancerous tumor, for ablation of cartilage in arthroscopy, or like medical procedures where relatively large amounts or relatively hard tissue is to be removed. At a wavelength of 2,100 nm, for example, this would require a power delivery of 40 to 130 watts to the tissue (about 50 to 150 watts at the laser head).
In a variety of surgical procedures involving the cutting of tissue with a laser beam, it is desirable to cut the tissue relatively quickly. In order to cut certain types of tissues at a relatively high rate, the incident laser energy at the tissue site, at a wavelength of 2,100 nm, for example, should preferably be delivered in pulses having a duration of about 200 to 600 microseconds at a repetition rate of 5 to 60 Hertz.
With many types of commercially available laser devices suitable for tissue cutting in medical applications, the production of such high energy levels, or such rapid pulse repetition rates, with a single laser resonator or oscillator (i.e., flash lamp, cavity, and crystal) is difficult or impossible, especially with an excimer, a holmium:YAG, erbium:YAG, ruby, alexandrite or similar lasers having limited output energy level and repetition rate capability. Indeed, many of the commercially available laser devices that are suitable for tissue cutting cannot be operated for extended time periods at such high energy levels or such repetition rates without creating excessive heat or placing excessive stress on the laser device or optical fiber waveguide, which can lead to premature component failure.
Accordingly, it would be desirable to provide an improved system for employing suitable, commercially available laser devices for generating radiant energy at higher energy levels, longer pulse widths, or faster repetition rates for delivery to a tissue site. It would also be advantageous to provide an improved laser system that can accommodate plural laser heads or laser devices for delivering laser energy of different wavelengths in intermittent, or substantially continuous, joined pulses.
Preferably, such an improved system should accommodate the use of commercially available, pulsed or continuous laser devices of the following types: neodymium:yttrium aluminum garnet (Nd:YAG), erbium:yttrium aluminum garnet (Er:YAG), holmium:yttrium aluminum garnet (Ho:YAG), ruby, alexandrite, carbon dioxide, excimer lasers such as argon fluoride (ArF), xenon chloride (XeCl), and other pulsed lasers.
Such an improved system should preferably operate to subject the tissue to pulses of laser energy at a sufficiently high average power and/or repetition rate within a relatively short time span so as to produce the desired effect in the tissue. In particular, it would be desirable to raise the temperature of the tissue to a desired elevated level, notwithstanding the tendency of the tissue temperature to decay or drop over time. In this regard, it will be appreciated that the temperature of tissue that has been initially raised to an elevated temperature T.sub.o decreases approximately according to the following equation: EQU T.sub.t =T.sub.o e.sup.-t/k
where T.sub.o is the maximum elevated temperature to which the tissue has been raised by a preceding pulse, e is the natural logarithm base, t is any selected time period following the establishment of the temperature T.sub.o, k is the tissue thermal diffusion time constant, and T.sub.t is the resulting time-dependent temperature at the end of the time period t.
When tissue is subjected to an initial pulse of laser energy, the tissue temperature rises to a maximum temperature T.sub.o, and then the tissue temperature begins to decrease. As the tissue temperature is initially rising, it would be desirable to provide increased energy to the tissue. It is believed that the efficiency of the laser ablation by the tissue can be increased by subjecting the tissue to pulses of laser energy in a way that results in little or no time temperature decay between laser energy pulses. Accordingly, the time span between pulses should be relatively short, preferably much shorter than the tissue thermal diffusion time constant.
For example, when a target site of a typical tissue is elevated to an initial temperature of about 100.degree. C., the tissue temperature decays to about 97.degree. C. in 5 milliseconds, and it would be desirable to subject the tissue to a plurality of laser energy pulses within such a time period or within an even shorter time. Preferably, an improved system should accommodate the emission of energy pulses from two or more conventional, medical lasers within such a time period wherein the laser energy pulses have a typical temporal separation of less than 5 milliseconds--and a pulse width of about 200 to 600 microseconds. The pulse width may vary depending upon a specific application. For example, for the fragmentation of ureteral, kidney or gall stones, a pulse width of about 10 to 1000 nanoseconds may be desirable.
While laser energy from two laser sources can be delivered through two independent optical fibers as known in the art, it would be beneficial to provide such an improved laser energy delivery system which could deliver such substantial laser energy through a single optical fiber or a bundle of optical fibers, a single hollow waveguide or an articulated arm. Such a system could also operate with two or more different laser types for subjecting the tissue sequentially to laser energy of different wavelengths to produce different effects on the tissue, such as cauterization by one wavelength, and cutting or ablation by another.
The present invention provides an improved laser energy delivery system which can accommodate designs having the above-discussed benefits and features.